New telescope reveals white dwarf gluttony

A highly magnetised stellar corpse has been caught stealing matter from its companion star in unprecedented detail, reveals the first scientific data from the new Southern African Large Telescope (SALT). The measurements will help scientists understand how matter behaves under extreme magnetic fields.

Researchers led by Darragh O’Donoghue of the South African Astronomical Observatory (SAAO) in Cape Town used a high-speed camera on the telescope, called SALTICAM, to measure the brightness of a system called SDSS J015543.40+002807.

The system is made up of an ordinary star and a white dwarf, the dense core left over after a star like the Sun burns out. Like other such pairs of stars, the white dwarf has a magnetic field that allows it to steal matter from its companion.

This white dwarf is unlike those in most other systems, however. For unknown reasons, it has an extremely strong magnetic field – millions of times stronger than that of our Sun.

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Sudden change

This field channels matter from its partner onto its poles, creating a bright spot at each pole. Because SALTICAM takes brightness measurements several times per second, it caught the spots winking off and on as the white dwarf passes behind its companion in its orbit, as seen from Earth.

This kind of sudden change in brightness is rarely seen in white dwarf systems, says Paula Szkody at the University of Washington in Seattle, US, who is not a team member. That is because the weaker magnetic fields of most systems do not produce such bright spots – these outshine the white dwarf and its companion combined.

“It’s a really a remarkable light curve,” Szkody told New Scientist. “We just didn’t have this kind of information on this type of system before. It’s a way of looking at what happens to matter under these extreme conditions.”

The telescope employs an unusual design in an effort to save money on construction and maintenance costs. Rather than moving its mirror on a giant motorised mount to follow celestial targets, the 11-metre primary mirror is held at a fixed angle relative to the ground. Its camera and other instruments then move to track objects as they drift across the sky.